This research is concerned with the perception and processing of visual stimulus structure by humans. Stimulus structure involves relations between attributes within sets of stimuli, and configural properties of individual stimuli. Four major areas of research are: (1) Asymmetry of selective attention to attributes. In classification tasks, with speed and accuracy measured, some pairs of attributes show that one can be selectively attended with irrelevant variation in the other, but not conversely. Attributes likely to produce asymmetric attention are visual location, color, and properties such as texture which define form or size, but in turn are not defined by form or size. (2) Contingent information processing, especially contingent classification. Contingent classification occurs when one stimulus property defines a subset of stimuli such that the task is changed from a many-to-two classification to alternative two-to-two discrimination tasks, without the primary or contingencing dimension giving any response information. The primary dimension can also be moved forward in time to precue the set of two stimuli, in which case the pertinent question is what properties of the stimuli are most easily cued. (3) Configural effects in perception. Lines imbedded in figures are often more easily discriminated than the lines in isolation. This object superiority effect may be a function of line orientation, and it may also become an inferiority effect depending on the actual configurations used. Further, either the superiority or the inferiority may be eliminated if either the lines or context are precued as in contingent classification. (4) Methodological issues. Experiments contrasting results of the classification tasks with the same-different task help clarify the nature of selective attention. Also, each type of task allows strategy options that change one task to the other, so that the two task types may allow strategies different from those intended by the experimenter. This research is directly relevant to developmental processes, both normal and abnormal. In addition, it is relevant to adult mental health problems, notably schizophrenia, because such patients have special difficulty dealing with relational or structural properties of stimuli.